Fated by Midnight: Married to the Nether Lord

At a college reunion Emily Carter is drugged and loses consciousness. She wanders into the wrong room, where Damien Black is lying badly wounded. She confuses him with the antidote she needs and takes the elixir he carries inside his body, the thing his strength runs on. Five years later Damien comes looking for the elixir. Emily has no memory of that night. She mistakes him for the blind date she was expecting and pulls him straight off to register a marriage. Then the people who set her up at the reunion start to reappear, and her memories come back with them. She learns that she was tricked that night, that she lost her virginity, and that she gave birth to a daughter afterwards. Damien helps her get the girl back. The girl's bloodline awakens, and that gives away who Damien is. Emily works out that he was the wounded man from the reunion night.